Flare stack

ABSTRACT

A Coanda flare is described that provides improved operation of a flare stack. A Coanda flare comprises a gas flue having a flare end, a Coanda body disposed adjacent the flare end to form an annular gap between the Coanda body and the flare end of the gas flue, such that gas emitted from the flare end passes through the annular gap and around the Coanda body, and a gas flow adjustor for adjusting pressure on gas flowing through the annular gap. The gas flow adjustor may be provided by a dual acting piston, which may for example be sensitive on one side to waste gas pressure, and on the other side to a control pressure.

BACKGROUND

U.S. Pat. No. 6,960,075 describes a flare stack operating on the Coandaprinciple with an adjustable gap for flare gas to pass through. This wasdone to allow the gap to be closed off by the Coanda body at lowpressures to avoid having to purge the gas flue. While this product hasserved its purpose, the device was designed to provide constant backpressure on gas in the flue. This meant that with different gascompositions, the resulting flame had different, and sometimesdeleterious, characteristics. For example, with heavy ends and constantpressure, the flame may shorten and become more turbulent. Turbulentflow close to the flare tip can cause premature erosion of the flaretip. The spring also caused a risk of chatter at the gap, and henceincreased noise.

SUMMARY

A gas flare is described that provides improved operation of a flarestack. A gas flare comprises a gas flue having a flare end, a bodydisposed adjacent the flare end to form a gap between the body and theflare end of the gas flue, such that gas emitted from the flare endpasses through the gap and around the body, and a gas flow adjustor foradjusting the gap, hence the pressure on gas flowing through the gap. Inone embodiment, the body is adjustable and connected to the gas flowadjustor. Pressure or flow adjustment allows adjustment for gascomposition and therefore tailored flame characteristics. The gas flowadjustor may be provided by a dual acting piston, which may for examplebe sensitive on one side to waste gas pressure, and on the other side toa control pressure. The body may be a Coanda body, and gap may beannular.

There is also provided a method for operating a gas flare in which gaspressure of gas flowing through the gap of a gas flare is adjusted toprovide desired flame characteristics. Thus, for example, in the case ofa heavier mix of gas, the gap size may be decreased to increase flowrate and extend the flame beyond the metal of the flare tip

These and other aspects are set out in the claims, which areincorporated here by reference.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of a gas flare stack will now be described with reference tothe figures, in which like reference characters denote like elements, byway of example, and in which:

FIG. 1 is a section through a Coanda flare; and

FIG. 2 is a schematic of a gas flow adjustor for a Coanda flare, inwhich a solid line with arrows indicates a primary flow line, a linewith repeated hash marks indicates instrument tubing and a wavy lineindicates flex hose.

DETAILED DESCRIPTION

In the claims, the word “comprising” is used in its inclusive sense anddoes not exclude other elements being present. The indefinite article“a” before a claim feature does not exclude more than one of the featurebeing present.

Referring to FIG. 1, there is shown an embodiment of a flare stack 100of the Coanda type, which has a Coanda body 10 disposed adjacent theflare end 11 of a gas flue 12. Gas flue 12 receives gas from a waste gassource 17 (FIG. 2), for example, an oil-gas separator, through pipeconnection 13. Pipe connection 13 and the gas flue 12 are secured to oneside of a frame 15 that is secured on its other side to a gas flowadjustor, which in the embodiment shown includes a dual acting cylinder20. The double acting cylinder 20 incorporates a dual acting piston 19having a lower piston face 22 that responds to pressure in a lowerchamber 23 of the dual acting cylinder 20 and an upper piston face 24that responds to pressure in an upper chamber 25 of the dual actingcylinder 25. The dual acting piston 19 is connected to the Coanda body10 through shaft 21. The Coanda flare stack 100 is mounted inconventional fashion on a support structure.

The Coanda body 10 responds to gas pressure in the gas flue 12 bylifting off the flare end 11 to form an adjustable annular gap 18between the Coanda body 10 and the flare end 11 of the gas flue 12. Gasemitted from the flare end 11 passes through the adjustable annular gap18 and around the Coanda body 10. The annular gap 18 is controlled bythe gas flow adjustor, which in the embodiment of FIG. 2 is provided bycontrolled pressure on either side of the dual acting piston 19. Theweight of the Coanda body tends to close the gap, but the device can beset with a minimum gap such as 0.100 inches by providing an uneven shapeto the mating surfaces of the Coanda body and flare tip. The unevenshape may be provided by any suitable means such as small bumps on theend of the gas flue.

FIG. 2 shows a gas flow adjustor for two Coanda flare stacks 100. Thedescription of the gas flow control system will be described for justone of the two Coanda flare stacks 100. In any flare facility, there maybe one, two or more Coanda flare stacks 100, such as six or eight. Asshown in FIG. 2, waste gas to be flared is provided through line 26 andconnection 13 to the gas flue 12. The waste gas is also provided to thedual acting cylinder 20 so that the lower face 22 of the dual actingpiston 19 is acted upon by the gas pressure of the gas to be flared. Theflare gas line 26 is connected to the cylinder 20 by gas line 28. Gasline 28 ends with flex hose connected into lower chamber 23 of the dualacting cylinder 20. The upper face 24 of the dual acting piston 19 ofthe flare stack 100 is acted upon by gas from a separate pressure source30 connected to piston 20 by gas lines 31 and 32. Gas line 32 terminatesin flex hose 33 that connects to the upper chamber 25 of the dual actingcylinder 20. The pressure in the upper chamber 25 of each dual actingcylinder 20, hence on the upper faces 23, is adjustable by a pinch valve34, although other means of adjusting the pressure could be used. Inthis embodiment, the pressure source 30 used is LPG (liquid petroleumgas), which may also used to supply pilot burners 35 with fuel throughline 37. Pressure in the line 37 may be controlled by a pinch valve 39.The pilot burner 35 is any suitable burner known in the art. It will beunderstood that other pressure sources 30 could be substituted,depending on the desired configuration.

Increasing pressure from the waste gas source 17 acting against thelower face 22 of the dual acting piston 19 acts to increase the gap 18,while increasing pressure against the upper face 24 of the dual actingpiston 19 acts to reduce the gap 18. In one embodiment, there is adeadband where pressure in the gas flue from the waste gas does notincrease the gap. The device thus has a deadband from 0 lb up to someset pressure where lift off of the Coanda body occurs. As the pressureof gas to be flared increases beyond the set point, for example 1 lb to5 lb of pressure, the force against the lower face 22 causes the gap 18to increase, in order to accommodate the higher rate of flow. Aspressure in the gas flue increases, without control pressure, the gaptends to increase until the gap is at a maximum. A gap range of0.51-0.550 inches may be used in some embodiments. The pressure at whichthe gap reaches a maximum can be set by the designer, as for example 10lb. FIG. 2 also shows a flame arrestor 36 on the waste gas line 26, andvarious ball valves 38 for controlling flow of gas in the gas lines.

In the embodiment of FIG. 2, the control pressure against the upper face24 tends to close the gap, and thus increase gas flow. The controlpressure is thus chosen to maintain a desired rate of flow of the gas tobe flared to achieve certain flare characteristics, such as flame speed,flame pattern, and flame length. The control pressure may be adjusted byoperation of valve 34 manually or by an automatic system. With higherflow rates, the flame will lengthen. By increasing the flame length, forexample, the heat released will not be concentrated on the Coanda body10, which helps reduce the damage caused by heat exposure, and arelatively flat orifice velocity is obtained, which reduces noiselevels. Thus, a mix of flare gas with larger amounts of propane, andrelatively less methane, tends to burn with a shorter flame and moreturbulence, which results in more damage to the flare tip, as comparedwith a lighter gas having more methane, at a given gap size. In the caseof propane, therefore, the gap size may be decreased to increase flowvelocity and extend the burn beyond the Coanda body.

The control pressure may also chosen to achieve a certain level ofstability for the Coanda body 10, and the noise level emitted by theflare stack 100. The noise level can be reduced significantly bychoosing a control pressure that allows gas to escape from the gap 18 atsub-sonic speeds. In addition, by having a regulated pressure actingagainst one side of the dual acting piston 19, ball flutter due to highfrequency vibration of the Coanda body 10 is reduced. Not only does thisincrease the efficiency of the flare stack 100, it also reduces thenoise as these vibrations would otherwise be transferred to the flamefront since the flow of gas would be disrupted by a changing gap 18.Favorable results have been obtained by using a control pressure between2 and 15 psig, and with the gap 18 adjustable between about 0.05 inchesand 0.5 inches, where 0.05 is the minimum width of the gap 18. Thecontrol pressure that may be used to close the gap may be important insome applications such as emergency shutdown or blow down situations.

In an array of gas flares, some gas flares may be isolated by turningoff gas flow to the gas flue while providing additional flow to theremaining gas flares.

Immaterial modifications may be made to the embodiments described abovewithout departing from what is claimed. For example, the gas flowadjustor may be provided by a dual acting diaphragm, or an electronicdevice responsive to a pressure or flow sensor on the waste gas flowline, flow sensors, noise sensors, flame sensors and/or other sensors.While in some embodiments a Coanda body is used for its desirable gasflow characteristics, the gas flare control may also be used with aplate or other mechanism that provides an adjustable gap. While theCoanda body is shown as adjustable, it is possible in some embodimentsto fix the Coanda body or other body and adjust the gap through othermeans such as movement of the upper end of the gas flue. Gas from thegas source may also be provided to both sides of the actuating cylinder20 for added control.

1. A gas flare, comprising: a gas flue having a flare end; the gas fluebeing connected to a source of waste gas; a body disposed adjacent theflare end to form a gap between the body and the flare end of the gasflue, such that gas emitted from the flare end passes through the gapand around the body; and a gas flow adjustor connected to the gas flueand to the body to adjust the gap and control gas flow through the gap.2. The gas flare of claim 1 in which the gap is annular.
 3. The gasflare of claim 2 in which the gas flow adjustor is responsive to wastegas pressure and to a control pressure from a pressure source.
 4. Thegas flare of claim 3 in which the gas flow adjustor comprises: a dualacting piston connected to the body for adjusting the annular gap, thedual acting piston being responsive on a first side of the dual actingpiston to waste gas pressure and on a second side of the dual actingpiston to the control pressure.
 5. The gas flare of claim 1 in which thepressure source has an operating pressure range that includes 2 psig and15 psig.
 6. The gas flare of claim 3 in which the pressure source is asource of LPG.
 7. The gas flare of claim 6 further comprising a gaspilot connected to the pressure source.
 8. The gas flare of claim 2 inwhich the annular gap has an adjustment range that includes gaps ofabout 0.05 inches and about 0.5 inches.
 9. The gas flare of claim 2 inwhich the body is a Coanda body.
 10. The gas flare of claim 3 in whichthe body is a Coanda body.
 11. The gas flare of claim 4 in which thebody is a Coanda body.
 12. A method of controlling a gas flare connectedto a waste gas source, comprising: providing a gas flare with anadjustable gap between a body and a flare end of a gas flue; supplyingthe gas flue with waste gas from the waste gas source; and controllingthe pressure of gas flowing through the adjustable gap by controllingthe gap size.
 13. The method of claim 12 in which the body is anadjustable Coanda body.
 14. The method of claim 13 in which controllingthe pressure of the gas flow comprises providing a control pressure froma gas source to a first side of a pressure responsive control device.15. The method of claim 14 in which the pressure responsive controldevice is a piston or diaphragm linked to the adjustable Coanda body.16. The method of claim 15 in which: the pressure responsive controldevice has a second side; and waste gas pressure is provided to thesecond side of the pressure responsive control device.
 17. The method ofclaim 16 in which the gap is adjusted to provide a desired flamecharacteristic.
 18. The method of claim 17 in which the desired flarecharacteristic is selected from a group consisting of flame speed, flamepattern, flame length, Coanda body stability, and noise level.
 19. Themethod of claim 12 in which the gap is adjusted to provide a desiredflame characteristic.
 20. The method of claim 19 in which the desiredflare characteristic is selected from a group consisting of flame speed,flame pattern, flame length, adjustable body stability, and noise level.